In WDM optical networks, which are based on the optical transparency offered by erbium doped fiber amplifiers (EDFA's) a number of signal channels can be transmitted simultaneously and routed independently. In such a network, it is desirable to ensure that when channels are added or dropped, which can result from changes in provisioning or network reconfigurations, as well as from network faults, the performance of the surviving channels will be not impaired. Because of the saturation characteristics of EDFA's this requires readjusting the EDFA gains to maintain channel power levels within acceptable limits.
Changes in provisioning and/or network reconfiguration can give rise to added or dropped channels, but these changes can in principle be predicted and corrected for by the network control and management (NC&M) system. However, it remains necessary to detect conditions arising from system faults because these cannot be controlled or predicted by the NC&M system. Such faults are most likely to result from channel failures or degradation in the optical transmission path. When these two problems happen, it is critical they be detected so that proper measures can be taken.
Monitoring the power of only one channel or the total power of all the channels does not provide sufficient information to determine the appropriate corrections. Variations of upstream optical losses or the output powers of upstream amplifiers which may occur cannot be distinguished from dropped/added channels but call for very different corrective actions. The proper way is to determine the number of channels present. One possibility is to demultiplex the channels (either in the transmission path or after a tap) and detect the presence of each channel independently. However, this method is complicated and expensive.